Annealing of silica to reduce the concentration of isolated silanols and peak tailing in reverse phase liquid chromatography

Non-porous, colloidal silica particles were annealed at three different temperatures, 800, 900 and 1050 °C. The adsorption of lysozyme, a probe of surface roughness, was consistent with progressively reduced surface roughness as temperature increased. The heat treated silica particles were rehydroxylated and then used to pack UHPLC columns. The cationic protein lysozyme was used to probe silanol activity, which exhibited progressively less tailing as the annealing temperature increased. FTIR spectroscopy confirmed that the abundance of isolated silanols on the surface was reduced by annealing at 900 °C or 1050 °C. FTIR also revealed that there was markedly increased hydrogen bonding of the isolated silanols to neighbors after rehydroxylation. These results combine to support the hypothesis that (a) isolated silanols on silica cause tailing in RP-LC and (b) nonplanar topography gives rise to isolated silanols.     

Determination of glass transition temperature of polyimides from atomistic molecular dynamics simulations and machine-learning algorithms

Glass transition temperature (T_g) plays an important role in controlling the mechanical and thermal properties of a polymer. Polyimides as an important category of engineering plastics have wide applications because of their superior heat resistance and mechanical strength. The capability of predicting T_g for a polyimide a priori is therefore highly desirable in order to expedite the design and discovery of new polyimide polymers with targeted properties and applications. Here we explore three different approaches to either compute T_g for a polyimide via all-atom molecular dynamics simulations or predict T_g via a mathematical model generated by using machine-learning algorithms to analyze existing data collected from the literature. Our simulations reveal that T_g can be determined from examining the diffusion coefficient of simple gas molecules in a polyimide as a function of temperature and the results are comparable to those derived from data on polymer density versus temperature and actually closer to the available experimental data. Furthermore, the predictive model of T_g derived with machine-learning algorithms can be used to estimate T_g successfully within an uncertainty of about 20 degrees, even for polyimides yet to be synthesized experimentally.     

Projection-forcing multisets of weight changes

Let F be a finite field. A multiset S of integers is projection-forcing if for every linear function ?:Fn→Fm whose multiset of weight changes is S, ? is a coordinate projection up to permutation and scaling of entries. The MacWilliams Extension Theorem from coding theory says that S={0,0,…,0} is projection-forcing. We give a (super-polynomial) algorithm to determine whether or not a given S is projection-forcing. We also give a condition that can be checked in polynomial time that implies that S is projection-forcing. This result is a generalization of the MacWilliams Extension Theorem and work by the first author.     

Oxidative Coupling of Terminal Rhenium Pnictide Complexes

The isolation of rhenium(V) complexes with terminal phosphide and arsenide ligands was achieved upon decarbonylation of rhenium(III) pnictaethynolates. One‐electron oxidation of the pnictide complexes yielded Pn?Pn (Pn=P, As) coupling products, which were spectroscopically and crystallographically characterized. Computational bond analysis suggests that these complexes are best described as {Pn₂}⁰ complexes that are stabilized by donor–acceptor interactions with the metal and a pyrazole ligand.     

Improved charge transfer multiplet method to simulate M‐ and L‐edge X‐ray absorption spectra of metal‐centered excited states

Charge transfer multiplet (CTM) theory is a computationally undemanding and highly mature method for simulating the soft X‐ray spectra of first‐row transition metal complexes. However, CTM theory has seldom been applied to the simulation of excited‐state spectra. In this article, the CTM4XAS software package is extended to simulate M_2,3‐ and L_2,3‐edge spectra for the excited states of first‐row transition metals and also interpret CTM eigenfunctions in terms of Russell–Saunders term symbols. These new programs are used to reinterpret the recently reported excited‐state M_2,3‐edge difference spectra of photogenerated ferrocenium cations and to propose alternative assignments for the electronic state of these cations responsible for the spectroscopic features. These new programs were also used to model the L_2,3‐edge spectra of Fe^II compounds during nuclear relaxation following photoinduced spin crossover and to propose spectroscopic signatures for their vibrationally hot states.     

Diffractive-optics-based beam combination of a phase-locked fiber laser array

A diffractive optical element (DOE) is used as a beam combiner for an actively phase-locked array of fiber lasers. Use of a DOE eliminates the far-field sidelobes and the accompanying loss of beam quality typically observed in tiled coherent laser arrays. Using this technique, we demonstrated coherent combination of five fiber lasers with 91% efficiency and M2=1.04. Combination efficiency and phase locking is robust even with large amplitude and phase fluctuations on the input laser array elements. Calculations and power handling measurements suggest that this approach can scale to both high channel counts and high powers.     

Phase ripple correction: theory and application

Spectral phase ripple associated with novel dispersive devices can distort broadband optical signals. We present a digital postprocessing algorithm to correct for this distortion by exploiting the static deterministic nature of the ripple. This algorithm is demonstrated with empirical data for several systems employing chirped fiber Bragg gratings (CFBGs). We employ this technique in a photonic time-stretch system incorporating CFBGs, improving the signal fidelity by 9 dB. Simulations and experiments show that this algorithm, which can be reduced to a simple interpolation and matrix multiplication, also mitigates additive noise. We see that the act of distortion correction yields signal fidelity superior to that of an ideal dispersive element.     

Addition of bromine chloride and iodine monochloride to carbonyl-conjugated, acetylenic ketones: synthesis and mechanisms

The reactions of 3-butyn-2-one (1), 3-hexyn-2-one (2), and 4-phenyl-3-butyn-2-one (3) with bromine chloride (BrCl) and iodine monochloride (ICl) in CH(2)Cl(2), CH(2)Cl(2)/pyridine, and MeOH are described. The data show that the major products in CH(2)Cl(2) are (Z)-AM (anti-Markovnikov) regioisomers. With the exception of 3 and ICl, the (E)-AM regioisomers predominate when pyridine was added as an acid scavenger. Minor amounts of the M regioisomers were formed with 1 and 2 and BrCl. The percentage of M regioisomer increased significantly with 1 and BrCl in MeOH, but MeOH had little affect on the other reactions. Isolation and stability of the products are discussed. Detailed evidence for the structures of the products, involving a combination of MS, (1)H and (13)C NMR, and IR, is presented; HRMS analyses are provided as proofs for all of the products. The acid-catalyzed mechanism and the halonium ion mechanism are considered as possible pathways in the formation of the products.     

Permeability of Porous Media from Simulated NMR Response

Nuclear Magnetic Resonance (NMR) is an increasingly popular well-logging tool in petroleum industry because it is the only tool that attempts to estimate formation permeability. In this paper, spatially correlated porous media are generated. Permeabilities of these media are computed by the lattice Boltzmann method. NMR relaxation responses are simulated by a random walk technique and formation factors are computed by solving a Laplacian equation. The testing of commonly used NMR permeability correlations shows that three conditions should be met for the validity of these correlations. The surface relaxivity should not vary significantly. The formation factor should depend only on porosity. And the characteristic pore body radius should be proportional to the characteristic throat radius. The correlations are improved by including surface relaxivity and formation factor.